The present invention relates to methods and apparatus for analysis of surfaces, and more particularly to photogoniometric surface analysis.
In the development and manufacturing of surface coatings such as latex and oil based paints, varnishes, etc., a variety of analytical techniques are used for characterizing the surface morphology of the coatings. These techniques are useful in comparing different coatings, as well as in quantifying variations due to weathering and other effects.
One such analytical technique is photometric in nature and employs an instrument known as a "gloss meter." In this technique, a beam of collimated light is directed upon the surface under analysis at a selected incident angle, and intensity of light reflected at an angle of reflectance equal to the angle of incidence is measured. This light intensity reading is then used as a measure of the gloss characteristics of the surface being considered.
Other, photogoniometric, techniques are known wherein for a single incident angle, the intensity of reflected light is measured over a variety of reflectance angles so as to more clearly show the actual reflectance characteristics of the surface under study. The output of a photogoniometric instrument is a graph representing reflection intensity versus reflectance angle.
In general, such reflectance graphs display two major artifacts; a diffuse reflectance characteristic, and a spectral peak which is superimposed upon this diffuse reflectance characteristic. A mirror-like surface will display a very large, narrow spectral peak located at approximately the reflectance angle equal to the angle of incidence, while displaying a very low diffuse reflectance characteristic. A perfect diffuser, however, will essentially lack a spectral peak, hence the diffuse reflectance characteristic will predominate.
In analyzing such reflectance characteristic graphs, it is desirable that the spectral and diffuse portions be characterized independently of one another. Consequently, either an analysis technique must be devised which can analyze the two portions individually by examining the total reflectance characteristic as a whole, or else some method must be provided for deconvoluting the total reflectance characteristic into two separate characteristics, one carrying the spectral portion alone and another carrying the diffused portion alone.
Deconvolution of the reflectance curve could be easily done if the shape of either the spectral or diffuse portion were known with certainty. The unknown portion could then be easily isolated by merely subtracting the known spectral or diffuse portion from the total characteristic. It is known that a perfect diffuser in theory displays a reflectance characteristic which varies as the cosine of the reflectance angle. Furthermore, shape of this diffuse characteristic is independent of the incidence angle, although its magnitude may change. Consequently, if the diffuse portion of the curve were entirely ideal the deconvoluting of the two portions would be quite simple, requiring only that an appropriately scaled cosine curve be substracted from the total characteristics.
In practice, however, the actual diffuse portion of the curve will differ from an ideal cosine characteristic. The residual diffuse curve which will then appear in the spectral portion upon subtraction of the cosine curve will perturb the characterization of the spectral reflectance portion of the surface under analysis.